Based On The Discrete Fourier Transform Of The Grid Harmonic Measurement And Analysis

The measurement and analysis of harmonics is the precondition and base of harmonics suppression. Since the Fast Fourier Transform (FFT) was presented, the harmonics measurement based on Fourier Transform which requires synchronous sampling has been used widely. Synch-sampling is an ideal AC sampling method. It may appear the deviation (sampling synchronous deviation) because of the limitation of minimum timing unit of timer and the fluctuation of signal frequency. The deviation will result in spectral leakage and picket fence effect which will degrade the accuracy of harmonics determination. So how to reduce the error caused by sampling synchronous deviation has already been studied by numerous scholars.After have studied deeply in the FFT theory, the thesis gives an improved FFTalgorithm--half-wave symmetric FFT which is based on the half-wave symmetriccharacteristic of the power system signals. The algorithm of half-wave symmetric FFT omits the even harmonics because the half-wave symmetric signals only have odd harmonics. As a result, the computation time of the improved algorithm of half-wave symmetric FFT is half of the algorithm of the traditional FFT.The number of samples in the traditional synchronous sampling method is fixed in order to use FFT algorithm. As the variations of the signal frequency, the real number of samples in one cycle changes with the fluctuation of signal frequency which will lead to sampling synchronous deviation. This paper proposeda new AC sampling method--variable N synchronous sampling, where thenumber of samples is calculated by the real frequency. The new AC sampling method almost eliminates the sampling synchronous deviation if the sampling frequency is high enough. The key problem in the new AC sampling method is the precise measurement of the signal frequency. So it introduces a simple,reliable way which can be used in real-time conditions to calculate the signal frequency,.